1. Field of the Invention
The present invention relates to an image capturing apparatus and a method for controlling the image capturing apparatus, and more particularly to an image capturing apparatus including an image sensor in which pixels each including a plurality of divided photoelectric conversion units are two-dimensionally arranged, and a method for controlling the same.
2. Description of the Related Art
Conventional image capturing apparatuses perform automatic exposure control based on, for example, a control line diagram as shown in FIG. 9. In FIG. 9, Open, F4 and F8 indicate the aperture size of the diaphragm. Open is the maximum aperture size of the diaphragm, and the aperture size of the diaphragm becomes smaller as the value of F number increases. Also, 1/60, 1/500 and 1/2000 indicate shutter speed, and the charge accumulation duration becomes shorter as the value decreases. In FIG. 9, when the luminance of the subject is high, the amount of light entering the sensor is suppressed by increasing the shutter speed to shorten the charge accumulation duration and/or by reducing the aperture.
With the recent trend toward smaller image sensors, small aperture blur due to diffraction is becoming more prominent, and thus there is a need to adjust exposure control on the high luminance side by control of the charge accumulation duration if possible. However, in the case where an image signal is periodically output when capturing moving images, as shown in FIG. 10, in each period tf, if the charge accumulation duration to of each pixel is short, the duration tb during which accumulation is not performed will be long. Accordingly, motion resolution decreases particularly when a moving subject is captured. As used herein, motion resolution means the smoothness of changes in moving images. For example, if the motion resolution decreases, the smoothness of subject motion in moving images decreases, resulting in the motion of the subject appearing like individual frames.
To address this, according to a conventional technique disclosed in Japanese Patent Laid-Open No. 02-241281, when a subject with high luminance is captured, short accumulation is performed, thereafter long accumulation is performed by adjusting a liquid crystal (LCD) shutter, and a signal obtained by short accumulation and a signal obtained by long accumulation are synthesized, so as to not have a non-accumulation duration.
Also, Japanese Patent Laid-Open No. 2006-135479 discloses a technique in which a decrease in motion resolution when capturing a subject with high luminance is reduced by, when the luminance of the subject becomes high, controlling exposure with an ND filter and then controlling the shutter speed.
With the conventional technique disclosed in Japanese Patent Laid-Open No. 02-241281, however, there is a difference in the amount of motion blur during the accumulation duration between the signal read out through short accumulation and the signal read out through long accumulation, and as a result, the synthesized image may be seen as two images.
The conventional technique disclosed in Japanese Patent Laid-Open No. 2006-135479 is also problematic in that it requires the ND filter, which increases the manufacturing cost of the image capturing apparatus as well as increasing the size of the image capturing apparatus.